Developing the Sport Horse: The Importance of Hydration

During intense exercise, horses can lose up to 10 to 15 liters (about 2.6 to 3.9 gallons) of sweat per hour.

Photo: Stephanie L. Church, Editor-in-Chief

As we move through the height of our summer competition season, it is not uncommon to have multiple days of severe heat and humidity—you know, the days when you sweat standing still.

But heat and humidity can be harder on your horse than it is on you. Most of us have worked to motivate a sluggish horse through the final jump off or Day 3 of a competition, but why does this happen? Was our training off the mark? Perhaps they need a different energy source in their feed? Or was it that energy-zapping heat and humidity? Let's take a close look at the role of sweating in the horses' ability to cool and how extreme heat and humidity can affect their ability to perform.

What happens to horses exercising in the heat?

As the horse begins to work, heat is produced as a by-product of muscle contraction at a 4:1 ratio. As the body temperature climbs and adrenaline levels increase, sweat glands respond by producing a hypertonic (highly concentrated) salt solution that coats the hair. Under normal circumstances horses cool by evaporative cooling (when the sweat coats the hairs and as air flows over them it pulls the moisture and the heat off the horse) and convection (when blood vessels near the skin dilate and allow the transfer of heat from the blood into the air). The movement of air over their body is paramount to both of these mechanisms.

During intense exercise, horses can lose up to 10 to 15 liters (about 2.6 to 3.9 gallons) of sweat per hour. Once their body temperature reaches greater than 42°C (107.6°F), the respiratory system kicks in to help "blow off" some of the extra body heat (approximately 15% of the body heat can be dissipated via respiration).

How does heat and humidity affect horses' ability to perform?

Perhaps the single most important calculation when determining the risk to your horse during hot and humid conditions is the "heat stress index," or HSI. It's important to consider temperature and percent humidity together to explain their full impact on an athlete's ability to perform. The higher the HSI, the harder it will be for your horse to effectively cool himself.

In humid conditions, the air is already saturated with moisture. This reduces evaporative cooling, allowing sweat to cling to the hair like a hot blanket and causes a rapid increase in body temperature. Horses that are struggling with overheating will often be "inverted" (when the respiratory rate is higher than the heart rate over the course of one minute) or seen to "pant."

Once fluid loss (dehydration) has reached 5% of body mass (approximately 20 to 25 liters [about 5.3 to 6.6 gallons] in your average saddle horse) there is a noticeable reduction in performance. Dehydration levels above 5% cause the athlete to feel ill as organ and brain function are impaired. It is also interesting to note that dehydrated athletes sweat less than normally hydrated horses (as the body tries to conserve any fluids it can). This further reduces the cooling mechanisms of the body, starting up a vicious cycle that can only lead to an overheated and severely dehydrated horse if not corrected early.

How can we improve our athlete management under these circumstances?

An ounce of prevention is worth a pound of cure!

Make a plan, Discuss the day's heat stress index with everyone at the barn so that all team members are aware of the risk and what they need to watch for over the course of the day. Early recognition is key.

Teach horses at home to drink electrolyte solutions or accept syringed electrolyte supplementation. Electrolytes can also be supplemented in the grain rations. Remember that horse sweat is a highly concentrated salt solution—there are approximately 10 grams of electrolytes per liter of sweat. Offer water frequently in between classes and ensure buckets are topped up when the horse returns to their stall.
Remember: Not all electrolytes are created equal, and different electrolytes are designed for different types of work. A Thoroughbred sprinting on the track requires different support than a sport horse competing over three to four days, in the same way a track star is managed differently from a marathon runner. Your safest bet is to discuss your options with your regular veterinarian and include them in the discussion of how to support the horses through a hot and humid competition.

Ensure horses (and riders) get frequent breaks out of the sun and have access to cool water and stable fans. If you notice a horse showing signs of heat stress (soaked in sweat, inverted respiratory pattern, dull eyes) get the horse into the shade and start cooling immediately while someone contacts the attending or show veterinarian.

For veterinarians and riders familiar with endurance horses, cooling horses becomes second nature with experience and education. The horse industry is littered with fallacy. Start by hosing or sponging cold water down the neck over the jugular veins, along the belly and in between the hind legs (look for the big dilated veins). The repetitive application of cold water from ears to tail is the key to removing excess heat from the body. Continue to apply water to the body until the water coming off is not gaining heat. The addition of air movement will enhance heat removal from the body (natural breeze or fans can be used).

Heat stress in horses can be very serious with potentially long-lasting impact on the ability of your horse to exercise during the heat. So take care to reduce the risk of heat stress for your horse during exercise, particularly during hot and humid weather.

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